Asthma is a clinical syndrome characterized by reversible airway obstruction, airway inflammation and hyperresponsivesness to a wide variety of stimuli. However, the pathogenesis has remained poorly defined. Certain microoganisms have been implicated in the exacerbation of asthma, particularly virus. Several studies have shown that infection with M. pneumoniae results in increased bronchial hyperresponsivesness and airflow obstruction in non-asthmatic patients that can last several years after infection. In addition, an animal model of airway disease similar to asthma can be produced with mycoplasma pulmonis.